This invention relates generally to an improved outlet screen assembly for a media mill, which is often referred to as a sand mill.
Sand milling is a proven, practical, continuous, high production method of dispersing and milling particles in liquid to produce a smooth, uniform, finely dispersed product. Some of the products for which the sand milling process is used include paints, inks, dye stuffs, paper coatings, chemicals, magnetic tape coatings, insecticides, and other materials in which milling to a high degree of fineness is required.
In a typical sand milling process, the material or slurry to be processed is introduced at one end of the processing chamber or vessel, and pumped through a small diameter grinding media while a rotor within the vessel agitates the media to mill and disperse small particles in the liquid or slurry being processed. Although the grinding media in years past was sand, currently a small manufactured product of steel, glass or other material is usually used.
The processed liquid exits from the vessel, but the grinding media must, of course, remain within the vessel. To accomplish this, the outlet structure of the mill typically includes a screen assembly which prevents media from leaving the vessel while processed liquid flows through the screen. U.S. Pat. No. 4,441,658 describes a cup-shaped assembly that leads to an outlet from the vessel. Other screen assemblies include segments forming a portion of a cylindrical wall. These screen components are typically formed of rods of small cross-section, which are welded at intersections to other small rods or struts. This in effect, creates a heavy mesh screen. A shortcoming of these welded constructions is that the screen becomes quickly worn because of the abrasive grinding media. This causes some of the strands of the screen to break or causes the openings between the strands to become large enough to allow passage of the grinding media. This requires early replacement of the screen. Also, such screen constructions are very difficult to clean, which often results in early replacement. The need for cleaning arises when a mill is unused for a period of time or when the mill is to be used for processing a different material.
The above referenced U.S. patent application Ser. No. 627,918 discloses a new and improved screen assembly for a sand mill wherein a plurality of rings or annular discs are stacked to form a cylindrical screen assembly. The rings are spared slightly to provide gaps or passages into which the liquid can flow, but the media cannot. Such a construction is much longer-wearing than the prior mesh type assemblies in that the radial thickness of the material is much greater. Also, the cross-section of the passages does not increase with wear. The ring screen is much easier to clean because the rings can be disassembled for that purpose. Further, the ring construction avoids the expensive welding process of the mesh type construction. In the arrangement shown in application Ser. No. 627,918, radially extending grooves or recesses are formed in the faces of the flat rings to provide the flow passages through the stack. The rings are illustrated as being held in the stack by a plurality of bolts extending through the rings.
The aforementioned U.S. application Ser. No. 746,440 discloses an improved arrangement wherein the spacing between rings is provided by four spaced pads on the face of the ring so that large radially extending slots or passges are formed between the pads. Also, the radial disc dimension of the rings is decreased in that bolts are no longer utilized for clamping the rings into a stack. Instead the discs are positioned by surrounding ribs and clamped as a group by an end plate. This application also discloses the screen assembly surrounding the downstream end of a horizontally oriented rotor. Such an arrangement is possible with the stack of rings because of the capability of the rings to withstand the abrasive wear of the grinding media, while still maintaining the same flow passage area.
In application Ser. No. 784,865, the ring-shaped screen elements are stacked on an internal ribbed carrier, which results in an assembly that is readily installed and removed as a unit from the outlet of a sand mill vessel. Another advantage of this approach and the others mentioned above, is that different stacks of rings providing different size flow passage, can be utilized on the same carrier.
While the foregoing described stacked ring screen assemblies, all provide significant advantages over the mesh type screen, a need still exists for improving the cost of manufacture of the rings. Also, improvements in the versatility in use of such screen assemblies is always desirable.